This invention relates to improvements in tractor-trailer fifth wheel couplings and, more particularly, to a low-friction, plastic slip plate or slip plate assembly which is placed between the fifth wheel top plate and the trailer bolster plate.
Such slip plates are known in the prior art, and are used to obviate the need for repeatedly lubricating the fifth wheel tractor-trailer coupling. See, for example, the disclosures of U.S. Pat. Nos. 4,169,635; 3,704,924 and 4,752,081. The slip plates are commonly held in place on the fifth wheel top plate by screws, rivets, adhesives, or preferably by welding with "weld fasteners".
The slip plates are commonly made from ultra high molecular weight polyethylene (UHMW PE) or ultra high molecular weight polystyrene (UHMW PS). These materials are preferred due to their exceptionally low coefficient of friction.
However, it has been found that slip plates from these materials are prone to damage if not protected somehow; the plastic is excellent for lubrication, but tends to damage relatively easily.
During the coupling between tractor and trailer, the mating bearing surfaces often impact each other suddenly and with considerable force. Such impact can cause tearing or shearing of the slip plate. To protect the slip plate during this coupling operation, metal "riser bars" or "skip plates" have been used in the prior art, each being positioned within a similarly shaped slot located in the interior region of the slip plate. For example, see U.S. Pat. Nos. 4,542,919 and 4,752,081. Such skip plates may be substantially flush with the surface of the slip plate, but preferably, as disclosed in U.S. Pat. No. 4,752,081, the thickness of such skip plates is slightly less than the thickness of the slip plate, i.e. the skip plates are recessed slightly below the upper surface of the slip plate. The skip plates absorb the impact of the trailer bolster plate, so that the slip plate is not damaged.
Although these skip plates, particularly as described in U.S. Pat. No. 4,752,081, offer improvements over what was previously available, nevertheless there remains room for improvement. One problem in particular is that the metal skip plates are more difficult to install and replace than is desirable, as well as being more expensive than is desirable. Another problem is that as the plastic slip plate wears down, the metal skip plates may become exposed, producing a metal-on-metal situation which may cause undesirable friction, and which thus reduces the benefit of having a plastic slip plate in the first place.
The outer areas of the plate also need additional impact and wear resistance. In the past, this has been dealt with by metal edges on the slip plate, as in U.S. Pat. No. 4,542,912, for example.